The present invention relates to a method for separating a reaction exhaust gas from production of carbon fiber and an apparatus used for separation of the gas, which exhaust gas is generated in a reaction furnace during production of carbon fiber, more particularly vapor-grown carbon fiber formed by thermal decomposition of an organic compound in a reducing atmosphere containing hydrogen; to a method for treating exhaust gasses and an incinerator used for treatment of the gasses, which gasses include the reaction exhaust gas and/or a thermal treatment exhaust gas generated during thermal treatment, including firing and graphitization, performed in a post-process; and to a method for producing the carbon fiber.
Carbon fiber is produced from a variety of raw materials, and fine carbon fiber is produced through a method in which an organic compound such as methane, ethylene, benzene, or toluene is thermally decomposed at 800-1,300xc2x0 C. in a thermal-decomposition furnace containing a reducing gas such as hydrogen or carbon monoxide, by use of a transition metal such as iron serving as a catalyst; i.e., a seed.
Specific examples of methods for producing carbon fiber include:
(1) a method for producing carbon fiber in which super-fine powder of a transition metal is distributed on a substrate in a thermal-decomposition furnace and used as seeds (Japanese Patent Application Laid-Open (kokai) No. 103528/1977);
(2) a method for producing carbon fiber in which a transition metal compound such as ferrocene is vaporized and introduced into a thermal-decomposition furnace to thereby form super-fine powder of a transition metal, and the powder is used as seeds (Japanese Patent Application Laid-Open (kokai) No. 54998/1985);
(3) a method for producing carbon fiber in which a transition metal such as iron is directly vaporized in a thermal-decomposition furnace to thereby form super-fine powder, and the powder is used as seeds (Japanese Patent Application Laid-Open (kokai) No. 291497/1986); and
(4) a method for producing carbon fiber in which a transition metal compound such as ferrocene is diffused or dissolved in an organic compound serving as a raw material, and the resultant mixture is introduced into a thermal-decomposition furnace to thereby form super-fine powder of a transition metal, and the powder is used as seeds (Japanese Patent Application Laid-Open (kokai) No. 180615/1983).
Japanese Patent No. 2778434 discloses a method for producing carbon fiber, in which an organic compound containing a transition metal such as iron, serving as a catalyst, is dissolved in a raw material liquid such as benzene, and the resultant solution is sprayed on the inner wall of a reaction furnace heated at 800-1,300xc2x0 C., to thereby thermally decompose the material. Specifically, a transition metal compound such as ferrocene, serving as a catalyst, is dissolved in a liquid organic compound such as benzene, and the resultant solution is sprayed on the inner wall of a reaction tube, serving as a thermal-decomposition furnace, by use of hydrogen serving as a carrier gas, to thereby form seeds and thermally decompose the organic compound. As a result, crude carbon fiber of fine fibrous shape is produced. (Hereinafter the above process will be referred to as xe2x80x9cthe first process.xe2x80x9d)
The thus-produced carbon fiber or the reaction furnace contains flammable gases including a carrier gas such as hydrogen, and hydrocarbon generated in a side reaction (hereinafter the flammable gases will be collectively referred to as xe2x80x9creaction exhaust gasxe2x80x9d), and thus the gas must be separated. A reaction exhaust gas which is separated from carbon fiber in a reaction furnace is collected with relative ease, but a reaction exhaust gas contained in carbon fiber, or in other words, captured between filaments of the carbon fiber, is difficult to separate.
Conventionally, a reaction exhaust gas is separated from carbon fiber containing the reaction exhaust gas by means of the following methods: (1) a method in which the temperature of a thermal-decomposition furnace is lowered after completion of reaction, and the inside of the furnace is substituted by nitrogen gas, to thereby separate the reaction exhaust gas; and (2) a method in which a recovery can is provided in a lower portion of a thermal-decomposition furnace, and carbon fiber containing a reaction exhaust gas is recovered in the can and the inside of the can is substituted by nitrogen gas, to thereby separate the exhaust gas.
However, when carbon fiber is industrially produced, in the above method (1), reaction or recovery is carried out batchwise, which is disadvantageous in terms of efficiency. In addition, the temperature of a thermal-decomposition furnace must be lowered, which is unsatisfactory in consideration of energy efficiency.
In the above method (2), a large recovery tube is required, due to low bulk density of carbon fiber, which results in high cost.
In the methods (1) and (2), the produced carbon fiber has a very low bulk density of 0.001-0.005 g/cm3 as measured immediately after production, which means a large volume of space between fibers. Thus, gas held in such space cannot be completely removed from the carbon fiber, and may directly accompany the fiber product.
In addition, the carbon fiber is detrimentally difficult to handle due to its low bulk density.
A recovered reaction exhaust gas is flammable and explosive, since the gas predominantly contains hydrogen. Therefore, conventionally, the gas is diluted in a blower in order to reduce the concentration of hydrogen below the range causing explosion, and then released in the air.
The crude carbon fiber produced in the reaction tube in the first process is usually scraped off and collected. The collected carbon fiber contains non-reacted organic substances, non-fibrous carbides, and tar, and therefore, in the next process the carbon fiber is thermally treated in a non-oxidative atmosphere. For example, the carbon fiber is subjected to thermal treatment such as firing and graphitization in a closed furnace as disclosed in Japanese Patent Application Laid-Open (kokai) No. 60444/1996, in a non-oxidative atmosphere of nitrogen, helium, or argon at a temperature which varies depending on required properties of a final product. (Hereinafter the above process will be referred to as xe2x80x9cthe second process.xe2x80x9d)
An exhaust gas generated in the second process predominantly contains inert gasses such as argon and nitrogen. In addition, the exhaust gas contains naphthalene, anthracene, and high-molecular weight substances such as tar, and thus the gas is difficult to combust. (Hereinafter the gas will be referred to as xe2x80x9cthermal treatment exhaust gas.xe2x80x9d)
Since the thermal treatment exhaust gas is difficult to combust, there was no other way than releasing it as is.
The present invention contemplates provision of a method and apparatus for continuously separating a reaction exhaust gas from carbon fiber with ease and in a safe manner, which exhaust gas is generated in the first process during production of carbon fiber through the above-described vapor-growth method, as well as a method and apparatus for combusting and air-releasing exhaust gasses at low cost, which gasses include the flammable reaction exhaust gas and a thermal treatment exhaust gas which is generated during thermal treatment in the second process and is difficult to combust.
The present invention also contemplates provision of a method for producing carbon fiber, including the above methods and apparatus.
Particularly, exposure of an operator to an organic compound such as benzene is regulated by the Law on Industrial Safety and Hygiene. In addition, such an organic compound is poisonous, and thus must be prevented from being released in the air. Meanwhile, hydrogen, methane, and ethylene are flammable substances, and leakage thereof may cause explosion.
Tar is difficult to collect, because of its high viscosity. A method for condensing tar by use of activated carbon or for causing tar to be adsorbed by activated carbon requires large-scale handling equipment, and tar poses problems in relation to waste treatment.
Moreover, in consideration of hygiene, tar must be handled carefully. Tar is preferably incinerated for disposal, but tar per se cannot be incinerated, since it is a high-molecular weight substance and contains inert gasses.
In order to solve these problems, the present inventors have studied a method for combusting a reaction exhaust gas. However, a reaction in the first process needs to be terminated in order to carry out equipment maintenance, and at such times only the second process may be carried out. In thermal treatment in the second process, the rate of generation of a thermal treatment exhaust gas is not constant, since the amount of carbon fiber varies. Therefore, by means of only a method for combusting such exhaust gasses, stable incineration is not carried out, and backfire to a reactor and a thermal treatment apparatus occurs, permitting damage to, for example, the apparatus. In addition, when fire is caused to be extinguished for some reason, a combustion apparatus may be filled with a reaction exhaust gas of high concentration, which provides a problem at re-ignition.
Accordingly, the present invention provides a series of processes for producing fine carbon fiber. Namely, the present invention provides a method for producing carbon fiber comprising: thermally decomposing an organic compound in a thermal-decomposition furnace at 800-1,300xc2x0 C. in an atmosphere containing a reducing gas, by use of a transition metal or a compound thereof serving as a catalyst, to thereby obtain fine carbon fiber; separating a reaction exhaust gas contained in the carbon fiber from the carbon fiber; continuously subjecting the carbon fiber to thermal treatment such as firing or graphitization in a non-oxidative atmosphere; and incinerating a thermal treatment exhaust gas generated during the thermal treatment, and/or the separated reaction exhaust gas.
The present invention also provides a method and apparatus for treating exhaust gasses, which gasses include an unwanted exhaust gas which is generated during a process for producing carbon fiber and an exhaust gas generated during thermal treatment such as firing or graphitization in the second process.
Specifically, such a method and apparatus for treating exhaust gasses according to the present invention contemplate a first method and apparatus for industrially separating a flammable exhaust gas from carbon fiber predominantly containing the gas, in a safe and continuous manner, and a second method and apparatus for incinerating in a dedicated incinerator the separated reaction exhaust gas and a thermal treatment exhaust gas from the second process, such that the gasses are incinerated independently or in combination. According to the first method; i.e., a method for industrially separating a flammable exhaust gas from carbon fiber during production of carbon fiber, a packed layer of carbon fiber is formed, the carbon fiber being produced at an outlet side of a reaction furnace of vapor-grown carbon fiber in the first process; an inert gas is caused to flow upward from the lower side of the packed layer; and the packed layer is compressed. According to the first apparatus used for separating the exhaust gas from vapor-grown carbon fiber, there are provided, as shown in FIG. 1, a separation tank 1 in which a packed layer of vapor-grown carbon fiber is formed; a compression chamber 3 provided at the lower portion of the tank, the chamber comprising a compression cylinder 2 and an inert gas inlet 7; and a shut-off valve 4 which enables switching between compression and exhaust.
According to the second method; i.e., a method for incinerating in a dedicated incinerator the separated reaction exhaust gas and a thermal treatment exhaust gas from the second process, there are provided the following:
(1) a method for treating an exhaust gas comprising incinerating an exhaust gas generated in the production process of vapor-grown carbon fiber (hereinafter referred to as xe2x80x9cexhaust gas of vapor-grown carbon fiberxe2x80x9d) of vapor-grown carbon fiber in a dedicated incinerator; and
(2) a method for treating an exhaust gas comprising mixing exhaust gasses; i.e., a flammable reaction exhaust gas and a thermal treatment exhaust gas which is difficult to combust and which is generated during firing or graphitization, and incinerating the gasses simultaneously, or incinerating either of the gasses in the incinerator. The above methods (1) and (2) are further defined as follows:
(3) a method for treating an exhaust gas of vapor-grown carbon fiber, in which the gas is combusted after being ignited by a flame of a pilot burner which is maintained by use of a flammable gas. In consideration of safety, the second method is preferably drawn to:
(4) a method for treating an exhaust gas of vapor-grown carbon fiber, in which a reaction exhaust gas in a reaction exhaust gas supply pipe is purged into the incinerator and incinerated upon completion of thermal reaction of vapor-grown carbon fiber, or in which a thermal treatment exhaust gas in a thermal treatment exhaust gas supply pipe is purged into the incinerator and incinerated upon completion of thermal treatment of the fiber;
(5) a method for treating an exhaust gas of vapor-grown carbon fiber, in which backfire-preventing apparatuses are provided in the supply pipes of a reaction exhaust gas and a thermal treatment exhaust gas leading into the incinerator, so as to enhance safety of a production apparatus; and
(6) a method for treating an exhaust gas of vapor-grown carbon fiber, in which flames of the pilot burner and a primary burner are monitored at all times, and when the flames are caused to be extinguished, supply of a reaction exhaust gas is switched to a release-to-air pipe, to thereby stop supply of the exhaust gases into the incinerator and enhance safety of the incinerator.
In order to attain any one of the above-described methods (1) through (6), the following incinerator is provided:
(7) an incinerator for processing an exhaust gas of vapor-grown carbon fiber, which comprises a primary burner for supplying a reaction exhaust gas, an auxiliary burner for supplying a thermal treatment exhaust gas, and a pilot burner for igniting the exhaust gasses by use of a flammable gas.
As used herein, the term xe2x80x9ccarbon fibersxe2x80x9d refers to annual-ring-form carbon fiber of multi-layer structure having a diameter of 0.01-5 xcexcm, which fiber is vapor-grown in a thermal reaction furnace according to the first and second method for treating an exhaust gas.